Thermostatic switches have long been used to protect motors, generators, transformers and like electrical components by breaking contact between the component and the power supply during an elevated ambient temperature and by re-establishing contact between the component and the power supply after the ambient temperature has cooled to a safe level. Although there are many different prior art switch designs, the switch design of concern generally can be be said to include a fixed contact mounted so as to face towards a planar member of the switch, and an elongated, electrically conductive bimetal blade, connected, at one of its ends, to the planar member with the other of its ends freely extending towards the fixed contact. Contact is made and broken, within the switch, by a movable contact, connected to the freely extending end of the bimetal blade, and the fixed contact. The bimetal blade flexes from its attached end, in response to a temperature change from the ambient, between an unstressed and undeformed state and a stressed and deformed state. In one such state, the movable contact is spaced from the fixed contact and in the other such state, the movable contact is located against the fixed contact to respectively establish circuit open and closed conditions of the switch.
A specific example of a prior art thermostatic switch, that incorporates the structure generally described above, is one in which the fixed contact and the bimetal blade are respectively mounted on a pair of contact strips that are in turn mounted in a non-conductive case with the strips insulated from one another. In such a switch design, the planar member is the contact strip that mounts the bimetal blade. Another example of a switch is one in which the blade is mounted on the basewall of an electrically conductive can and the fixed contact is mounted on an electrically conductive lid that is insulated from the can. In this type of switch design the planar member is the basewall of the conductive can. These two specific examples of thermostatic switch designs, while common, are by no means exhaustive of all prior art designs of a thermostatic switch. It is understood that these examples are mentioned for exemplary purposes and are not intended to limit the scope of applicability of the present invention.
A common method of attaching the end of the bimetal blade to the planar member, in any of the switch designs described above, as well as other switch designs that are well known in the art, is by means of a weld button and a series of tack welds that penetrate the planar member, the bimetal blade and the weld button. In a switch design that incorporates a non-conductive case, the tack welds penetrate one of the contact strips, the bimetal blade and the weld button. After the welding process is completed, the contact strip carrying the bimetal blade, is installed in the case with the basewall of the case underlying the contact strip. In most, if not all prior art switch designs, the tack welds form a circular pattern that extends around the periphery of the weld button. As can be appreciated, the rise and fall of the ambient temperature and the consequent flexure of the bimetal blade, induces residual stresses to build up in the bimetal blade until the blade fails. Since the blade flexes from its attached end, failure normally occurs in the blade at the tack welds. Depending upon the actual design of the blade, this failure can occur between about 8,000 and 30,000 cycles of flexure. The purpose of the circular pattern of tack welds is to solidly attach the bimetal blade to the planar member and to thereby reduce the build up of residual stresses in the end of the bimetal blade. It has been found, however, by the inventor herein, that the tack welds crimp the blade, and as such, act as stress intensification sites. Thus, rather than reducing the build up of residual stresses, the pattern of tack welds increases the build up of residual stresses in the bimetal blade.
In accordance with the improvement of the present invention, a weld button and a weld are used in the connection of the bimetal blade to the planar member. However, in the present invention, unlike the prior art, the weld button is preloaded by a preload force so that the weld button bears against the attached end of the blade with a pressure that holds the blade in place. The weld penetrates the planar member, the weld button and the blade. The purpose of the weld is, however, to prevent relaxation of the pressure rather than, as in the prior art, to attach the bimetal blade to the planar member. As such, the weld is configured, in a manner well known in the art, to be strong enough to prevent relaxation of the pressure after removal of the preload force. Therefore, the blade is attached to the planar member essentially by the preloaded weld button instead of the weld. Although after repeated cycles of operation, the bimetal deforms at the specific point of the weld, the creation of a large, circular weakened area produced by the circular pattern of tack welds is therefore avoided. Thus, while it would be expected that a single weld would increase the stress intensification, the small area presented by the single weld, combined with the preloaded weld button actually reduces the build up of residual stress and increases the cyclical life expectancy of the blade. In this regard, it has been found the diameter of the weld should preferably be no more than 1/3 of the diameter of the weld button. Employing the type of attachment described herein the life of the bimetal blade can be extended to about 100,000 cycles. Experimentally, a blade attached in accordance with the present invention failed after about 1,750,000 cycles of flexure.